The invention relates to the method and a device for producing a substantially circularly cylindrical body, particularly a sintered metal blank, which consists of plastic material and which has at least one helical internal recess extending in the interior of the body, according to the introductory part of patent claim 1 or of patent claim 12. In addition, the invention relates to a sintered rod produced by the method according to the invention.
Such bodies are required particularly in the manufacture of drilling tools or drilling tool inserts of hard metal or ceramic materials. Through the helical course of the at least one internal recess, which serves in the finished drilling tool for the feed of coolant or lubricant to the cutting region, the drilling tool can be furnished with helical cutting grooves which are often of advantage for the provision of favourable cutting and material removal characteristics and consequently are desired.
It has previously been attempted to produce such sintered metal blanks or ceramic blanks by an extrusion method, in that the material consisting of sintered metal powder or ceramic powder and binder is forced through an extrusion nozzle which has a cross-section corresponding to the desired blank cross-section and further has at least one internally disposed core in the form of a pin which on extrusion of the plasticised material serves for formation of the internal recess extending through the entire blank.
The material issuing from the extrusion nozzle is usually very pressure-sensitive, i.e. the issuing blank deforms extremely easily in the case of external application of force. Since such deformations are no longer reversible and thus lead to blanks which are unusable at least in sections, it has been attempted to further develop the extrusion process so that the blank already has the helically extending cooling channels when issued from the extrusion nozzle. According to one proposal this is achieved in the manner that helically extending guide strips, which impose a twist motion on the issuing plastic material, are mounted at the inner circumference of the extrusion nozzle. Flexible threads with a cross-section corresponding with the cross-section of the internal recess to be produced are fastened in the cross-section of the extrusion nozzle, wherein the threads extend up to the outlet of the nozzle mouthpiece. Due to the flexibility of the threads these can follow the swirl motion or the swirl flow of the plastic material and thus generate the at least one internally disposed cooling channel in the blank.
According to a further proposal the nozzle mouthpiece and/or a hub formed in propeller shape, to which the aforesaid flexible or pliable threads are fastened, is set into rotary motion during the extrusion process, whereby again an externally smooth blank with internally disposed helical channels or recesses could be produced.
In the production of such tool blanks it is important that the angle of inclination of the at least one helical internal recess is kept constant over the entire length of the blank and within closely toleranced limits. This is required because regular cutting grooves are ground into the tool blank after the sintering process. This grinding is carried out by largely automated machines, so that in the case of imprecise production of the helical internal recesses an uncontrolled high reject rate can result. In that case it has to be taken into consideration that tools with fully hardened metal cutting parts are used inter alia because the high loading capability of the material, particularly the torsional stiffness, has to be utilised. In order to ensure this the internal recess must not extend too close to the cutting groove, which cannot, however, be effectively excluded in the case of inaccurate production of the helical internal recess. In the case of the afore-described attachments for production of blanks with internally disposed helical recesses it is accordingly necessary to monitor as accurately as possible the extrusion tool and/or the sintering devices for the extrusion worm orxe2x80x94if presentxe2x80x94for the twist-generating bodies during the extrusion process and to adapt to the material throughput. This has the consequence that relatively lengthy changeover and setting times are required at the extrusion tool with the result that conventional methods are economically usable primarily for large batch production. Disproportionately high machine setting costs result for small batch production or for production of drilling tools with greater nominal diameters, whereby the economics of the production method are called into question.
The invention accordingly has the object of creating a method and a device of the aforesaid kind by which the blank or blanks of the kind described in the introduction can be produced more economically and, as before, with high precision.
This object is met with respect to the method by the features of patent claim 1 and with respect to the device by the features of patent claim 12.
According to the invention the blank is produced as before in an extrusion process, which is distinguished by a high economy by virtue of high possible throughput rates. The extrusion is carried out so that the at least one inwardly disposed recess is extruded rectilinearly, which has the advantage that the production parameters for the extruding, i.e. the extrusion speed, the material throughput, etc, no longer have an effect on the course of the internally disposed recesses. Instead, a body extruded with substantially rectilinear internal recesses is cut to a predetermined length, i.e. cut to length, and subjected in the cut-to-length state to a special deforming process which is based on the principle of a rolling motion engaging the extruded rod over the entire length thereof. The arrangement is in that case such that the speed of the rolling motion changes linearly and constantly over the length of the extruded rod or body, wherein the inclination of the helically extending internal recess is determined by way of the gradients of the speed distribution of the rolling motion. With the method according to the invention the extruded body is uniformly twisted over its entire length and with maintenance of favourable, i.e. constant, support relationships, wherein a minimal deformation of the blank cross-section results due to the rolling motion taking place in that case. The consistency of the extruded extrusion material is thus of benefit to the attaching in accordance with to the invention. The extruded extrusion material is uniformly of viscous consistency so that by virtue of the friction surface arrangement a largely slip-free entrainment of the outer surface of the extruded blank is ensured. The accuracy of the course of the at least one internally disposed helical recess can thus be kept to a particularly high level.
Advantageous embodiments of the method and of the device for production of the substantially circularly cylindrical blank body with a helical internal recess are described in the subclaims.
It has proved that with support of the blank along a line it is already possible to produce the at least one helical cooling channel without unacceptably high deformations of the blank cross-section. A particularly simple device for performance of this development in terms of method forms the subject of claims 13 and 14. Such a device merely requires a support surface and a surface mounted parallel thereto to be pivotable about an axis perpendicular to the support. The inclination of the at least one helical internal recess can be determined by way of the absolute magnitude of the relative pivot angle between the support and the friction surface arrangement. This inclination is directly proportional to the size of the pivot angle.
The deformation of the extruded blank during twisting can be kept within even closer limits by the development of the method and the device according to claims 4 and 15. The support of the extruded blank during twisting preferably takes place with a looping angle of substantially 180xc2x0, wherein in this case the external forces due to gravitational force can be kept to a minimum. A particularly simple construction of the device with a minimum of components and in that case at the same time a particularly gentle support of the deformation-sensitive extruded blank results with the development of the device according to patent claim 17. This device is accordingly suitable in particular measure for extrusion materials with a high proportion of plasticiser.
Since the blank tends to shorten during twisting it is of advantage if the surface formed from the film material or textile material is composed of a plurality of part surfaces which are disposed in a line axially along the axis of the body and between each of which a respective gap is provided. The film material or textile material can thus accompany the shortening of the blank rod without excessive force action on the blank, which is of further benefit to the production accuracy of the at least one internally disposed cooling channel.
If the drive device engages the side edges of the film material or textile material in accordance with claim 21 then a comparatively wide scope is created for the design of the drive device. The drive device can, through appropriate selection of the length of the film material or textile material or of a corresponding fabric, be placed in a desired region above the fabric bend, i.e. above the blank to be reshaped. Thus, many possibilities for the accommodation of a drive device are left to the constructer.
It has proved that in the case of use of a fabric connected to the side edges, four raising and lowering drives engaging at the corners are sufficient in order to uniformly twist the extruded blank over the entire length. Stepping motors, which are preferably program-controlled, are preferably used as drive units. The deforming can thus be set in simple manner and, for example, adapted to different nominal diameters of the drilling tool to be produced, whereby a minimum of re-equipping effort is needed.
Further advantageous embodiments are the subject of the remaining subclaims.